Conventional sheet-fed offset printing machines comprise an ink fountain, an ink fountain roller in fluid communication with the ink fountain, and a plurality of inking rollers for transferring ink to the printing plate located on the plate cylinder of the printing machine. The amount of ink required to be fed to the printing plate is dependent on a number of factors, such as the design of the printing machine and the inking requirements of the printing plate then in use. Different printing plates have different inking requirements, and different portions of the same printing plate may require different amounts of ink. Accordingly, conventional printing machines include a plurality of adjustable ink metering elements disposed zonally along the width of the printing machine, that is, transversely across the ink fountain roller. The operator adjusts the ink metering elements until the proper amount of ink is fed to the various zones of the printing plate.
After the printing machine has been operated continuously for a long period of time, the amount of ink on the inking rollers reaches a steady state. In this steady state, the amount of ink on each zone of each inking roller remains relatively constant, with exactly the amount of ink being fed to the inking roller as is required and taken off by the printing plate. In this state, high-quality prints having a consistent ink density are produced by the printing machine.
Prior to reaching the steady state, however, the printed sheets produced by the printing machine are not consistently marked by the printing machine, and tend to be of poor quality. For example, during initial start-up of the printing machine, an ink gradient, or profile, is built up between the ink fountain roller and the downstream inking rollers. The printing machine must go through a large number of revolutions before the ink profile reaches a steady state condition. Accordingly, when starting up a printing machine after a cleaning, or when changing the printing plate to prepare a new print order, the initially printed sheets will be inconsistently and improperly inked.
The prior art has sought to provide a method for loading ink into the printing machine in advance of commencement of printing in an effort to mitigate the waste of time and sheets occurring before steady state is reached. For example, in DE 3,707,695 C2, a method for the defined production of an ink distribution is described. Between printing orders, the ink present in inking unit is reduced to a minimum amount by closing all of the zonally distributed ink metering elements. After the ink quantity in the inking unit has been minimized, the new distribution of ink required by the new printing plate is produced within the inking unit. In one embodiment of the disclosed method, the difference in ink quantity between two printing orders is determined by means of a computer, and is created by appropriate adjustment of the ink metering elements. The ink may be transported both from the inking unit to the ink fountain and in the reverse direction, i.e., from the ink fountain to the inking unit, as a result of the ink film thickness profile present in the printing machine. However, the method described in this reference is limited to the adjustment of the ink profile between two printing orders.
A preadjustment of an inking unit is described in DE 3,338,143 A1. In this reference, a method of filling an empty inking unit is described. The ink film thickness profile corresponding to the inking requirements of the particular printing plate loaded into the printing machine is produced on the inking rollers. The disclosed method is limited to the initial filling of the inking unit with ink, and the procedure for creating the ink profile within the empty inking unit may be complicated.
The foregoing references describe methods that are limited respectively to regulating the ink distribution in the printing machine between two printing orders and to regulating the ink distribution in a clean inking unit prior to commencement of printing. Moreover, the running-on steady state cannot be achieved precisely in either method, and the ink transport can be optimized only with difficulty. In addition, in certain cases where large differences between the intended ink flow and the actual ink flow are possible, the waste of printing sheets and printing time is actually increased, rather than decreased.
Accordingly, a need exists for a method of filling an inking unit with ink, the method suitable for use prior to commencement of printing or between printing orders. A need further exists for a method of filling an inking unit that allows for optimum preparation of the inking unit for a given print order, without significant complexity or expense.